1. Field of the Invention
The present invention relates to a charging pressure control apparatus for an internal combustion engine with a dual turbocharger system which can improve engine output characteristics at high altitudes.
2 Description of the Related Art
An internal combustion engine with a dual turbocharger system is shown in Japanese Patent Publication HEI 3-138,420 (which corresponds to U.S. Pat. No. 5,081,842).
The dual turbocharger system includes a first turbocharger operated at all intake air quantities and a second turbocharger operated solely at large intake air quantities. More particularly, an intake switching valve is installed in a portion of the intake conduit downstream of the compressor of the second turbocharger and an exhaust switching valve is installed in a portion of the exhaust conduit downstream of the turbine of the second turbocharger. When the intake switching valve and the exhaust switching valve are closed, operation of the second turbocharger is stopped and only the first turbocharger is in operation. When the intake switching valve and the exhaust switching valve are open, both the second turbocharger and the first turbocharger are in operation. To make the switching from "one-turbocharger-operation" to "two-turbocharger-operation" smooth, an exhaust bypass conduit is provided to bypass the exhaust switching valve and an exhaust bypass valve is installed in the exhaust bypass conduit. Before the intake switching valve and the exhaust switching valve are opened, the exhaust bypass valve is opened so that the second turbocharger is pre-rotated.
The exhaust bypass valve is also used as means for controlling the charging pressure in "one-turbocharger-operation" as described below. More particularly, the exhaust bypass valve is operated with a diaphragm actuator. The charging pressure is introduced to the diaphragm chamber of the diaphragm actuator. The diaphragm chamber also communicates with the intake line upstream of the turbocharger compressor or atmosphere via a duty control solenoid valve. By controlling the duty control solenoid valve, the degree of opening of the exhaust bypass valve is controlled whereby the amount of exhaust gas leaking to the second turbocharger turbine without flowing to the first turbocharger turbine is controlled so that the rotational speed of the first turbocharger and the charging pressure in "one-turbocharger-operation" are controlled.
The charging pressure control in "two-turbocharger-operation" is performed by a waste gate valve which is installed in a bypass conduit bypassing the first turbocharger turbine. The waste gate valve is operated by an actuator which in turn is operated by a duty control solenoid valve. By controlling the duty control valve, the degree of opening of the waste gate valve is controlled whereby the amount of exhaust gas bypassing the first turbocharger turbine is controlled so that the rotational speed of the first turbocharger and the charging pressure in "two-turbocharger-operation" are controlled.
In FIG. 15, full line L1 shows the charging pressure characteristic at low altitudes in accordance with the above-described conventional turbocharged engine. More particularly, after the engine starts-up, the intake pressure rises in accordance with an increase in the engine speed. When the intake pressure reaches a feed-back control beginning pressure Pc, the feed-back control of the duty control solenoid valve for the exhaust bypass valve starts so that the charging pressure is controlled to an objective pressure Po (constant in absolute pressure). Before the engine speed increases and the intake gas quantity Q finally reaches a predetermined value (for example, 5,500 l/min), the charging pressure is controlled by the exhaust bypass valve. When the intake air quantity reaches the predetermined value, the engine operation is switched from "one-turbocharger-operation" to "two-turbocharger-operation" accompanied by a sudden decrease in the charging pressure. Passing through the transitional range, the charging pressure is controlled to the constant pressure Po by the waste gate valve.
However, when the turbocharged engine is used at high altitudes, the charging pressure characteristic changes from full line L1 to broken line L2 (FIG. 15), because the intake air is lean in density, and the following problems are encountered:
(a) When the charging pressure changes from Pc to P0, the charging pressure increase is slow compared to that at low altitudes, which degrades the acceleration characteristic at high altitudes. PA0 (b) When the operation changes from "one-turbocharger-operation" to "two-turbocharger-operation", the charging pressure decreases to a greater extent at high altitudes than at low altitudes, which causes a large torque shock. (c) If the intake pressure does not reach the feed back control beginning pressure Pc at high altitudes (see broken line L3 in FIG. 15), the charging pressure cannot be controlled to the constant pressure P0, which will deteriorate the charging characteristic at high altitudes.